In today's automotive market, there exist a variety of propulsion or drive technologies used to power vehicles. The technologies include internal combustion engines (ICEs), electric drive systems utilizing batteries and/or fuel cells as an energy or power source, hybrid systems utilizing a combination of internal combustion engines and electric drive systems, and pure electric systems. The propulsion systems each have specific technological, financial, and performance advantages and disadvantages, depending on the state of energy prices, energy infrastructure developments, environmental laws, and government incentives.
The increasing demand to improve fuel economy and reduce emissions in present vehicles has led to the development of advanced hybrid vehicles, as well as pure electric vehicles. With regard to pure electrical vehicles, no ICE is required. Electric vehicles are classified as vehicles having only one energy source, typically a battery or a hydrogen-fed fuel cell. Hybrid vehicles are classified as vehicles having at least two separate energy sources, typically gasoline to feed an internal combustion engine and a battery system linked to an electric traction motor. Hybrid vehicles, as compared to standard vehicles driven by an ICE, have improved fuel economy and reduced emissions. During varying driving conditions, hybrid vehicles will alternate between separate power sources, depending on the most efficient manner of operation of each power source. For example, during most operating conditions, a hybrid vehicle equipped with an ICE and an electric motor will shut down the ICE during a stopped or idle condition, allowing the electric motor to propel the vehicle and eventually restart the ICE, improving fuel economy for the hybrid vehicle.
Hybrid vehicles are broadly classified into series or parallel drivetrains, depending upon the configuration of the drivetrains. In a series drivetrain utilizing an ICE and an electric traction motor, only the electric motor drives the wheels of a vehicle. The ICE converts a fuel source to mechanical energy to turn a generator, which converts the mechanical energy to electrical energy to drive the electric motor. In a parallel hybrid drivetrain system, two power sources such as an ICE and an electric traction motor operate in parallel to propel a vehicle. Generally, a hybrid vehicle having a parallel drivetrain combines the power and range advantages of a conventional ICE with the efficiency and electrical regeneration capability of an electric motor to increase fuel economy and lower emissions, as compared with a traditional ICE vehicle. In addition, hybrid vehicles can incorporate both series and parallel paths. Further, hybrids are often described as being either charge-depleting or charge-sustaining with reference to a battery pack. Charge-depleting hybrids can be charged off the electrical grid; thus, these hybrids share many of the characteristics of purely electric vehicles. In contrast, the batteries in charge-sustaining hybrids receive all of their electrical charging from the ICE and/or from regenerative braking.
Battery packs having secondary/rechargeable batteries are an important component of hybrid vehicle systems, as they enable an electric motor/generator (MoGen) to store braking energy in the battery pack during regeneration and charging by the ICE. The MoGen utilizes the stored energy in the battery pack to propel or drive the vehicle when the ICE is not operating. During operation, the ICE will be turned on and off intermittently, according to driving conditions, causing the battery pack to be alternatively charged and discharged by the MoGen.
State of charge (SOC) is a term that refers to the stored charge available to do work relative to that which is available after the battery has been fully charged. SOC can be viewed as a thermodynamic quantity, enabling one to assess the potential energy of the system. As can be appreciated, the state of charge (SOC) of the battery pack in a vehicle system such as a hybrid vehicle system is important in relation to vehicle efficiency, emissions, and power availability. For example, a vehicle operator or an onboard controller might utilize the SOC for purposes of regulating the operation of the battery pack.
It is known in the art to use a look up table to regulate a battery pack having parameters pre-computed based on a standard vehicle or an experimental vehicle. A standard vehicle is a reference vehicle other than the actual vehicle that is being operated. A difficulty with the prior art approaches is that they are either not vehicle specific, or lack a generalized approach to multiple parameter handling, thereby reducing the utility of the prior art systems. In addition, it is known in the art to use Coulomb counting to get an SOC value of a battery system. Coulomb counting is easily implemented, provided the initial SOC and the current efficiency is known precisely for all times and conditions. Because this is not normally the case, Coulomb counting tends to be an impractical solution.